U.S. Pat. No. 4,371,870, issued Feb. 1, 1983, discloses a display system arranged to operate as a back up for a primary liquid crystal display.
U.S. Pat. No. 4,562,433, issued Dec. 31, 1985, discloses an arrangement similar to the aforementioned U.S. Pat. No. 4,371,870.
U.S. Pat. No. 4,568,928, issued Feb. 4, 1986 is generally related to the above two references.